Ultraviolet light is classified into long-wavelength UV light (UVA), the wavelength of which is 320 to 400 nm, middle-wavelength UV light (UVB), the wavelength of which is 290 to 320 nm, and short-wavelength UV light (UVC), the wavelength of which is below 290 nm.
Among them, UVA and UVB reach the Earth's surface without being absorbed or scattered by the ozone layer and cause various harmful effects. Specifically, it is known that UVB induces erythema, bulla, etc. In addition, it has been clarified that UVA not only induces a suntan on the skin but also lowers systemic immune function and triggers skin cancer.
In order to prevent the harmful effects to the human body, various UV protection agents have been developed. In particular, titanium oxide is most frequently used in the fields of cosmetics, paints, and chemical fibers as the UV protection agent that is chemically and physically stable and highly safe.
The shielding of UV light with titanium oxide is achieved by the absorption and scattering of UV light.
(1) Absorption Effect
Titanium oxide has electrically a semiconductor structure, and the valence band and the conduction band are not continuous. Therefore, titanium oxide absorbs light of a wavelength that corresponds to the energy higher than the bandgap, which is the energy difference between the two levels. Titanium oxide mainly absorbs UV in the UVB region.
(2) Scattering Effect
The light scattering ability of titanium oxide is dependent on the relationship between the particle size and the wavelength of light. If the particle size of microparticulate titanium oxide is comparable to the wavelength of light, Mie scattering takes place and the maximum is reached in the vicinity of ½ wavelength. If the particle size is smaller than that (less than about 1/10 wavelength), Rayleigh scattering takes place and its scattering ability is inversely proportional to the fourth power of the wavelength.
Therefore, it is necessary to suitably control the particle size in order to maintain transmission in the visible region and effectively shield UV light.
In recent years, sufficiently small microparticulate titanium oxide (less than 100 nm) compared with the wavelength of visible light is predominantly used so that the scattering of visible light is low from the standpoint of transparency.
In patent literatures 1 to 4, for example, dispersions in which microparticulate titanium oxide is dispersed in an oil dispersion medium are disclosed.    Patent literature 1: Japanese Unexamined Patent Publication No. H09-208438    Patent literature 2: Japanese Unexamined Patent Publication No. 2002-80771    Patent literature 3: Japanese Unexamined Patent Publication No. 2004-2887    Patent literature 4: Japanese Unexamined Patent Publication No. 2004-35632